feat: 实现成绩管理系统核心功能

添加响应工具、错误处理中间件和数据库模型
创建用户、学生、课程和成绩相关服务
实现管理员、教师和学生控制器的基本功能
重构路由处理并优化数据库查询

backend/models/Score.js

@@ -0,0 +1,55 @@
+const db = require('../config/database');
+
+class Score {
+    static async findByStudentId(studentId) {
+        const sql = `
+            SELECT s.*, c.course_code, c.course_name, c.credit,
+                   u.name as teacher_name
+            FROM scores s
+            JOIN courses c ON s.course_id = c.id
+            JOIN users u ON s.teacher_id = u.id
+            WHERE s.student_id = ?
+            ORDER BY s.created_at DESC
+        `;
+        return await db.query(sql, [studentId]);
+    }
+
+    static async findDetailsById(scoreId, studentId) {
+        const sql = `
+            SELECT s.*, c.course_code, c.course_name, c.credit, c.semester,
+                   u.name as teacher_name, u.email as teacher_email,
+                   st.id as student_number, st.class as class_name
+            FROM scores s
+            JOIN courses c ON s.course_id = c.id
+            JOIN users u ON s.teacher_id = u.id
+            JOIN students st ON s.student_id = st.id
+            WHERE s.id = ? AND st.id = ?
+        `;
+        const rows = await db.query(sql, [scoreId, studentId]);
+        return rows[0];
+    }
+
+    static async create(scoreData) {
+        const { studentId, courseId, teacherId, score, gradePoint, gradeLevel, examDate, remark } = scoreData;
+        const sql = `
+            INSERT INTO scores (student_id, course_id, teacher_id, score, 
+            grade_point, grade_level, created_at, remark) 
+            VALUES (?, ?, ?, ?, ?, ?, NOW(), ?)
+        `;
+        // 注意：这里用 created_at 替代了 examDate，如果数据库有 examDate 列可以改回去
+        const result = await db.pool.execute(sql, [
+            studentId, courseId, teacherId, score, gradePoint, gradeLevel, remark
+        ]);
+        return result[0].insertId;
+    }
+
+    static async findByStudentAndCourse(studentId, courseId) {
+        const rows = await db.query(
+            'SELECT * FROM scores WHERE student_id = ? AND course_id = ?',
+            [studentId, courseId]
+        );
+        return rows[0];
+    }
+}
+
+module.exports = Score;